In the screen printing industry, advances in the art have resulted in the ability to create higher tensions in screens. While higher screen tensions are certainly beneficial, several problems associated with the higher tensions of modern screens have become apparent.
For example, higher screen tensions exacerbate problems of imbalanced tension in the warp and weft directions. Imbalanced tension typically occurs because the screen material stretches more in the warp direction than in the weft direction, creating distortion and ripples in the screen, especially towards the corners. Distortion accelerates screen mesh fatigue caused during the printing process. Increased screen tension also increases convex displacement of the screen frame which may result in imbalanced tension and fatigue. If tension is applied too rapidly, the stress may result in excessive strain followed by premature fatigue. If too much tension is applied to the screen in either the warp or weft direction, the screen fibers may also tend to neck, resulting in a severely weakened or unusable screen.
In tensioning and framing a screen, the screen is typically stretched over a rectangular frame. The screen mesh is commonly made of polyester or nylon. Stretching the screen over the screen frame can be done by hand, resulting in a number of individual operations to ensure that the screen is properly tensioned. Hand tensioning is time-consuming and sometimes results in poorly tensioned screens due to human error. Alternatively, the screen can be tensioned mechanically. For example, edges of the screen may be attached to rollers, and the rollers turned to create tension in the screen. When the screen is at the proper tension the rollers are then locked into place.
A screen can also be tensioned by continuous or sequential stretcher bars. However, the tensioning performed by these bars suffers from the same problems of imbalanced tensioning, rippling, and weakening of the screen material described above. The stretcher bars of the prior art do not provide for balanced screen tensions nor elimination of problems resulting from imbalanced tension as does the present invention.
A need has developed for a screen tensioning and framing device which provides for a well-balanced screen tension, thus avoiding problems such as screen and image distortion, and minimizing screen mesh fatigue. A need has also developed for a device which compensates for convex screen frame displacement when tensioning a screen, and which eliminates necking of screen fibers due to overtensioning from time and/or stress. A need has further developed for a screen which has a smaller ratio of displacement from the center to the perimeter of the mesh. The screen tensioning and framing device of the present invention solves these and other problems.